With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, and intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
Leaf seals are formed from sections of leaf material appropriately presented in order to create a seal surface from juxtaposed leaf edges of respective leaves in an assembly. Typically the leaves are arranged circumferentially about a rotating shaft in order to present the leaf edges and therefore the seal surface towards that shaft in order to provide a seal barrier. Typically, spacer members are provided between each leaf in order to correctly arrange the seal elements for presentation of the leaf edges and therefore the seal surface. These spacers may be independent components or integrally formed with each leaf. The leaf edges and so the seal surface effectively floats upwards and downwards relative to a rotating surface.
In a gas turbine engine, leaf seals may be used to form a seal between a static component and a rotating component, between two relatively rotating components, or even between two static components in order to maintain a relatively high pressure on one side of the seal and relatively low pressure on the other. FIG. 2, which shows schematically, for example, a cut-away perspective view of a portion of a leaf seal assembly 31 comprises leaves 32 extending from spacer elements 33 secured in a housing comprising a backing ring 34 with coverplates 35. The leaves 32 present leaf edges 36 towards a surface 37 of a rotating component generally rotating in the direction depicted by arrowhead 38. The leaves 32, and in particular the leaf edges 36 of the leaves 32 act against the surface 37 in order to create a seal across the assembly 31. Each leaf 32 is generally compliant in order to adjust with rotation of the surface 11 to ensure that a good sealing effect is created. The spacers 33 are generally required in order to ensure that flexibility is available to appropriately present the leaves 32 towards the surface 37 which, as illustrated, is generally with an inclined angle between them.
U.S. Pat. No. 6,267,381 proposes providing two sets of seal elements in tandem. For example, FIG. 3 shows schematically a longitudinal cross-section through a seal formed by annular leaf packs 56, 58. In each pack, leaves 46 extend between a stationary outer housing 44 to provide leaf edges that make wiping contact with shaft 42. The leaf packs are axially separated by a clearance 60. Coverplates 62 are located upstream and downstream of the leaf packs and separated therefrom by distances 64.
Leaf seals such as those shown in FIGS. 2 and 3 rely on their coverplates to block the bulk of the flow area through the leaf pack(s), flow area being strongly linked to seal leakage performance. The coverplates immediately upstream and downstream of the leaf pack(s) allow the direct flow area through the leaf pack(s) to be reduced, as the coverplates cover much of the interleaf gaps, which are generally largest close to the leaf root. Reducing the coverplate to leaf pack distance further decreases the seal leakage, as it reduces the amount of air that can flow radially outwards or inwards in the space between each coverplate and the adjacent leaf pack. Hence in order to minimise leakage it is conventionally desirable to minimise the coverplate to leaf pack distances.
However, such coverplates also influence the blow up and blow down performance of the seal (as discussed, for example in H. Nakane et al., The Development of High-Performance Leaf Seals, Trans. ASME, Vol. 126, April 2004, pp. 342-350). To control blow down and blow up, the ratio of the front coverplate to leaf pack distance and the rear leaf pack to coverplate distance needs to be controlled accurately. Excessive blow down can lead to accelerated leaf wear and excessive blow up leads to increased leakage.
Due to manufacturing tolerances that apply during leaf pack and coverplate manufacture, this creates a design conflict. In particular, if both distances are small (e.g. having a size which is similar to manufacturing tolerances) the leakage performance is good, but the gap ratio and hence blow up and blow down control are poor. Similarly if both gaps are large (e.g. having a size which is bigger than manufacturing tolerances) the blow up and blow down control is good, while the leakage performance is poor.